In the past, voids (air bubbles) generated in a normal hot blast (air) reflow processing during a reflow mounting process of large current device such as mounting process of power device and power module have been held in question. A method of allowing generation of any voids to be more reduced has been required.
FIGS. 13A and 13B are processing diagrams for showing the hot blast reflow processing according to a past example. Cream solder 8 shown in FIG. 13A is applied onto a pad electrode 4 of a substrate 5. The cream solder 8 is made by adding flux to solder powder so as to have suitable viscosity. It is applied onto the pad electrode 4 of the substrate 5 by a screen printer through a mask.
In the hot blast reflow processing according to the past example, the hot blast reflow processing is performed on the cream solder 8 and when the solder melts, voids 2 are generated in an interior thereof. Such a problem that when the molted solder 7 is cooled and solidified, these voids 2 remain in the solder as they are has occurred.
The following will schematically describe the voids generation in such a hot blast reflow processing condition that the cream solder 8 is applied onto the pad electrode 5 of the substrate 5 but the electronic component is not mounted, with reference to FIGS. 13A and 13B. The solder 3 shown in FIG. 13B is such a condition that after the hot blast reflow processing is performed on the cream solder 8 shown in FIG. 13A, the molten solder 7 is cooled so as to be spherically solidified by surface tension. Circular white portions in the drawing are voids 2 which are unexpectedly generated in the molten solder 7 and remain in the solder 3 after it is cooled and solidified. The voids 2 cause heat conduction effect to be missed in the power device or the like, so that this causes waste heat to bring about a change for the worse.
Regarding the above-mentioned reduction of the voids generation, patent document 1 discloses a soldering apparatus (vacuum reflow apparatus) having a vacuum evacuation function. This solder apparatus is provided with an exhaust valve, a vacuum pump and a treatment chamber. Under the condition in which the substrate is imported into the treatment chamber and the solder on the pad electrode of the substrate is melted, the exhaust valve is opened and the vacuum pump is driven so that the interior of the treatment chamber can exhaust to become temporarily vacuum condition. Such a vacuum condition allows any voids which remain in the solder as bubbles while the solder is melted to be removed by defoaming effect.
Patent Document 1: Japanese Patent Application Publication No. H09-314322.